The present invention relates to a phase and speed control system for synchronizing the rotational velocity and relative rotational phase of a pair of prime movers and, more particularly, to such a system for synchronizing speed and controlling the phase relationship between a pair of aircraft engines, each of said pair of aircraft engines having associated therewith a centrifugal governor.
It is highly desirable that the rotational velocity of the engines in a twin aircraft be synchronized in order to avoid excessive vibration. If the engines are operated at slightly differing speeds, vibrations generated by each engine alternately add to and subtract from the vibrations produced by the other engine to create vibrational "beats" which occur at a frequency equal to the difference in frequency between the vibrations generated by the engines. Even when the engines are synchronized precisely in speed, vibrations generated by each of the engines will be additive and subtractive at various points in the aircraft. This may result, for example, in the rearward, passenger portion of a craft experiencing a great deal more vibration and noise than the cockpit of the craft. By adjusting the relative rotational phase between the engines, it is possible to vary the position in the aircraft of the vibrational nodes such that the passangers in the craft may be provided with the smoothest and quietest conditions possible. Alternatively, when no passengers are in the aircraft, the rotational phase relationship between the engines may be adjusted to provide minimum noise and vibration in the cockpit.
In order to provide precise engine speed control, it is common to utilize a variable pitch propeller with the speed of the engine driving the propeller being adjusted by varying the pitch of the propeller blades. Adjusting propeller pitch, it will be appreciated, causes a corresponding change in the loading on the driving engine, with the result that the speed of the engine is adjusted. Typically, the pitch of the propeller blades is controlled by a centrifugal governor driven by the engine.
One such governor, known as a magnetic ball head governor, includes rotating flyweights which are connected by a spring biased linkage to a hydraulic valve. Hydraulic fluid passing through the valve is supplied to the hub of the propeller, through a hydraulic actuator mechanism, and determines the propeller pitch. Adjustment of the mechanical linkage that provides spring bias to the flyweights results in a coarse speed setting. As shown in U.S. Pat. Nos. 2,890,877, issued June 16, 1959, to Straznickas, and 3,955,165, issued May 4, 1976, to Stubbs et al, such ball head governors also typically include an electromagnetic coil which, when energized, alters the position of the flyweights during rotation. The flyweights of some governors are magnetized, in which case they are either attracted or repelled in dependence upon the direction of current through the electromagnetic coil. Other governors have non-magnetized flyweights, in which case they are attracted to the coil regardless of the direction of current. The level of current supplied through the electromagnetic coil in the ball head governor is selected to provide a fine adjustment of the speed of the engine.
A number of speed and phase control systems, incorporating ball head governors or similar governor mechanisms, have in the past been utilized to control the engine speed and relative phase in multi-engine aircraft. One such control system is disclosed in U.S. Pat. No. 2,232,753, issued Feb. 25, 1941, to Wilson. The Wilson system incorporates mechanical switches which, through a relay system, control hydraulically the pitch of a propeller driven by a slave engine to synchronize the slave engine speed and phase with that of a master engine. Mechanical switching arrangements are subject to wear and, therefore, are inherently unreliable. Additionally, the response time and accuracy of such systems may not be sufficient in all motor control applications.
U.S. Pat. No. 3,689,175, issued Sept. 5, 1970 , to Hartzell et al, discloses a system for phase and speed control of aircraft engines in which the speed and phase relationship of a slave engine is compared photo-optically to that of a master engine. A strobe lamp is strobed in synchronization with the rotation of the master engine and a slotted wheel, driven by the slave engine, is rotated adjacent to the strobe lamp. A photo-optical transducer on the opposite side of the slotted wheel senses the relative speed and phase differences between the slave engine and the master engine and adjusts a potentiometer which, in turn, controls a d.c. current applied to the control coil in the ball head governor.
U.S. Pat. No. 3,367,110, issued Feb. 6, 1968, to Lesson, discloses a system for controlling the speed and phase relationship between a slave engine and a master. The Lesson system is a digital system in which tachometer output pulses from the master engine and the slave engine are compared and pulse width modulated signals applied directly to the governor control coil in the slave engine governor. The response time of the coil is such that the governor does not react immediately to each pulse supplied to the coil, but rather responds to the average coil current. By adjusting the pulse width of the pulses applied to the governor coil, therefore, the average current is adjusted and control of the governor provided without the need for a digital-to-analog converter arrangement.
All of the control systems discussed above arbitrarily assign the status of slave engine to one or more of the engines and synchronize the speed and phase of the slave or slaves with an engine selected as the master engine. It will be appreciated that such an arrangement is somewhat undesirable in that should a malfunction occur in the master engine and the master engine shut down, the control system will then attempt to stop all of the slave engines.
U.S. Pat. No. 3,785,147, issued Jan. 15, 1974, to Leeson, discloses a digital synchronizing and phase control system or a pair of aircraft engines in which the engines are speed-synchronized and phase-controlled without assigning slave and master status to the engines. Tachometer output pulses from the engines are phase-compared and complementary pulse width modulated control signals are generated for application to the electromagnetic coils in the governors on both of the engines. Phase adjustment between the engines is accomplished by adding a variable d.c. biasing current to the pulse width modulated signal applied to one of the governor coils. Since both of the governor coils receive signals, the Leeson system may, in certain situations, cause the engine running at the higher initial rotational velocity to increase its speed, prior to obtaining speed synchronization. For example, both of the engines controlled by the Leeson system may be accelerated when system operation is initiated. As can be appreciated, this is highly undesirable in that the faster running of the engines may be initially operating at the upper safe operating speed of the engine and an engine overspeed condition may result. Additionally, should one of the engines fail, the system of Leeson will attempt to slow down the other of the engines. This is also highly undesirable. Finally, the circuitry used for controlling the phase between the engines is analog in nature and its operation may therefore be adversely affected by changes in ambient conditions, such as temperature.
Accordingly, it is seen that there is a need for a simple, reliable speed and phase control system for controlling a pair of aircraft engines in which both speed and phase control are accomplished digitally, in which engine overspeed conditions are precluded, and in which the operation of either of the aircraft engines is unaffected in the event that the other engine should be shut down.